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Fixed the handling of the sign of values from the ADC #1

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Merged
merged 1 commit into from
Jul 1, 2019
Merged

Fixed the handling of the sign of values from the ADC #1

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27 changes: 19 additions & 8 deletions src/SparkFun_Qwiic_Scale_NAU7802_Arduino_Library.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -197,7 +197,7 @@ uint8_t NAU7802::getRevisionCode()

//Returns 24-bit reading
//Assumes CR Cycle Ready bit (ADC conversion complete) has been checked to be 1
uint32_t NAU7802::getReading()
int32_t NAU7802::getReading()
{
_i2cPort->beginTransmission(_deviceAddress);
_i2cPort->write(NAU7802_ADCO_B2);
Expand All @@ -208,9 +208,20 @@ uint32_t NAU7802::getReading()

if (_i2cPort->available())
{
uint32_t value = (uint32_t)_i2cPort->read() << 16; //MSB
value |= (uint32_t)_i2cPort->read() << 8; //MidSB
value |= (uint32_t)_i2cPort->read(); //LSB
uint32_t valueRaw = (uint32_t)_i2cPort->read() << 16; //MSB
valueRaw |= (uint32_t)_i2cPort->read() << 8; //MidSB
valueRaw |= (uint32_t)_i2cPort->read(); //LSB

// the raw value coming from the ADC is a 24-bit number, so the sign bit now
// resides on bit 23 (0 is LSB) of the uint32_t container. By shifting the
// value to the left, I move the sign bit to the MSB of the uint32_t container.
// By casting to a signed int32_t container I now have properly recovered
// the sign of the original value
int32_t valueShifted = (int32_t)(valueRaw << 8);

// shift the number back right to recover its intended magnitude
int32_t value = ( valueShifted >> 8 );

return (value);
}

Expand All @@ -219,9 +230,9 @@ uint32_t NAU7802::getReading()

//Return the average of a given number of readings
//Gives up after 1000ms so don't call this function to average 8 samples setup at 1Hz output (requires 8s)
uint32_t NAU7802::getAverage(uint8_t averageAmount)
int32_t NAU7802::getAverage(uint8_t averageAmount)
{
unsigned long total = 0;
long total = 0;
uint8_t samplesAquired = 0;

unsigned long startTime = millis();
Expand Down Expand Up @@ -261,7 +272,7 @@ int32_t NAU7802::getZeroOffset()
//Call after zeroing. Provide the float weight sitting on scale. Units do not matter.
void NAU7802::calculateCalibrationFactor(float weightOnScale, uint8_t averageAmount)
{
uint32_t onScale = getAverage(8);
int32_t onScale = getAverage(averageAmount);
float newCalFactor = (onScale - _zeroOffset) / (float)weightOnScale;
setCalibrationFactor(newCalFactor);
}
Expand All @@ -281,7 +292,7 @@ float NAU7802::getCalibrationFactor()
//Returns the y of y = mx + b using the current weight on scale, the cal factor, and the offset.
float NAU7802::getWeight(bool allowNegativeWeights)
{
uint32_t onScale = getAverage(8);
int32_t onScale = getAverage(8);

//Prevent the current reading from being less than zero offset
//This happens when the scale is zero'd, unloaded, and the load cell reports a value slightly less than zero value
Expand Down
6 changes: 3 additions & 3 deletions src/SparkFun_Qwiic_Scale_NAU7802_Arduino_Library.h
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Original file line number Diff line number Diff line change
Expand Up @@ -161,8 +161,8 @@ class NAU7802
bool isConnected(); //Returns true if device acks at the I2C address

bool available(); //Returns true if Cycle Ready bit is set (conversion is complete)
uint32_t getReading(); //Returns 24-bit reading. Assumes CR Cycle Ready bit (ADC conversion complete) has been checked by .available()
uint32_t getAverage(uint8_t samplesToTake); //Return the average of a given number of readings
int32_t getReading(); //Returns 24-bit reading. Assumes CR Cycle Ready bit (ADC conversion complete) has been checked by .available()
int32_t getAverage(uint8_t samplesToTake); //Return the average of a given number of readings

void calculateZeroOffset(uint8_t averageAmount = 8); //Also called taring. Call this with nothing on the scale
void setZeroOffset(int32_t newZeroOffset); //Sets the internal variable. Useful for users who are loading values from NVM.
Expand Down Expand Up @@ -202,7 +202,7 @@ class NAU7802
const uint8_t _deviceAddress = 0x2A; //Default unshifted 7-bit address of the NAU7802

//y = mx+b
uint32_t _zeroOffset; //This is b
int32_t _zeroOffset; //This is b
float _calibrationFactor; //This is m. User provides this number so that we can output y when requested
};
#endif